Source Four Mini Incandescent is designed to use, and ships with, a 50W incandescent MR16 lamp. Some users wonder if they can use an LED retrofit MR16 lamp instead.
An incandescent MR16 lamp consists of a point source and a reflector, and the optics of the Source 4 mini are designed to accommodate this. While a bare LED MR16 lamp can nicely mimic an incandescent as a soft spot with an unmodified beam, they usually achieve this through different means - for example, having multiple sources, or using a plastic lens.
The results of using these lamps in a Source 4 Mini will be unpredictable, and ETC cannot recommend any particular model or style of lamp that will give the best results (although lamps with multiple sources will almost certainly give a very strange beam). Dimming performance will also suffer compared to a tungsten lamp, and gobo projection may well be impossible.
Another concern is heat - because tungsten halogen lamps can withstand very high temperatures, the S4 Mini is not designed with cooling in mind. Enclosing an LED MR16 in the fixture may not allow it to cool sufficiently, which may affect performance and lamp life.
While this is not an officially supported modification, you are of course free to try it out, but it will require trial and error until you find a solution that is suitable for your particular application.